fix(example): handle possible undefined results from Dictionary

[alex-okrushko]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https:/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

Adjust the example to handle the updated Dictionary, which now types as T | undefined.

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #
#1735

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for e0614f4 at https://previews.ngrx.io/pr1745-e0614f4/

[brandonroberts]

Any reason not to use .filter(Boolean)?

[alex-okrushko]

.filter(Boolean) doesn't work with strictNullChecks
PLAYGROUND LINK